Facebook announced Slingshot this week, an ephemeral messaging app that should compete with Snapchat. Photo: Mashable

The social network relaunched Slingshot in the US on Tuesday, an app for sending disappearing photo and video messages similar to those sent via Snapchat.

Users can send photos or videos up to 15 seconds long using Slingshot, and once those messages are viewed and cleared by the recipient, they disappear from the recipient's phone for good. The only catch: Users are required to respond with an image or video in order to "unlock" those they receive. For example, if you receive a message from a friend, you'll need to send a message back to that friend before you can open what they sent you.

To unlock messages, images or videos, users will first have to share something themselves. Photo: Supplied

This is a way for Facebook to encourage — or force — users to share more frequently. It also alleviates the "pressure" that comes with creating and sharing content, Slingshot design lead Joey Flynn said.

"What we found is that you don't feel the need to respond immediately," Flynn told Mashable. "It's more [like], I want to share what I'm up to whenever I can, and then they're going to feel almost no pressure to share back whatever they're doing because it's a shared experience."

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Slingshot users don't need a Facebook account to use the app. Instead, accounts are tied to a user's cellphone number; once a username is created, contacts can be imported from the phone's address book or the user's Facebook account.

If this news sounds familiar, you're onto something. For a short while one week ago, Slingshot launched in the App Store — and disappeared just a few hours later, as if it were an ephemeral message itself.

It turned out that last week's launch was accidental, and before anyone could capture much more than a screen grab, the app was gone.

Slingshot product manager Will Ruben wouldn't specify what caused the accidental launch, during which the app appeared in countries like Australia and Russia before it was taken down. Apps can launch prematurely when developers mistakenly set the wrong date or time zone for the app's launch, and it's also possible someone inside Facebook simply pressed the "publish" button too early.

Regardless, it seems Slingshot will be around for good following Tuesday's announcement, which comes a little more than a month after the company killed off Poke, a separate Snapchat competitor that never gained traction following its launch in 2012. Facebook has been working on Slingshot for just more than six months, Ruben said, and the app was born from a company-wide hackathon in December.

Flynn and Slingshot's engineering lead Rocky Smith worked on the app for three days straight, developing a prototype that employees tested over the holidays. The team has since grown to 10 people.

While the ephemeral messaging app is similar to Snapchat, Slingshot has a number of small features that set it apart. Unlike Snapchat, photos sent through Slingshot don't self-destruct after 10 seconds. Images and videos can only be viewed once, but the duration of that viewing period is up to the recipient.

During this viewing period, a recipient could take a screen grab of an image, preserving it after the image is wiped from the app. The message creator will not receive an alert if someone they send a message to captures a screen shot, Ruben said, as happens in Snapchat.

Message security will be important for Facebook, as the company is notorious for its large collection of user data. Ruben said that once you view and discard a message, you won't be able to see it again on your phone. Once a message has been viewed by each recipient (users can send Slings to an unlimited number of friends at a time), Facebook will then delete it from its servers after a seven day grace period.

The weeklong period is for users who report images as inappropriate. If a message sits unopened for 30 days, Facebook considers that message read, and will delete it from its servers a week later.

Facebook has taken an interest in ephemeral messaging for years. In January, a Forbes cover story detailed a 2012 meeting between Snapchat CEO Evan Spiegel and Facebook CEO Mark Zuckeberg in which Zuckerberg threatened to crush Snapchat with Poke. One year later, Spiegel turned down a $3 billion acquisition offer from Zuckerberg after Poke failed to live up to Zuckerberg's promises.

Slingshot is the latest in what is becoming a robust collection of standalone apps owned by Facebook. Zuckerberg promised investors more standalone apps in January, and he has lived up to his word thus far in 2014. Facebook acquired messaging app WhatsApp in February, and has released two apps of its own in Slingshot and the news reader app Paper, which it unveiled in January.

Slingshot is available to US users on both iOS and Android beginning Tuesday.

Mashable is the largest independent news source covering digital culture, social media and technology.